Photoresists are fundamental materials related to. They are light-sensitive materials, composed of a polymer, a sensitizer, and a solvent. Each element has a particular function. The polymer changes its structure when it is exposed to radiation. The solvent allows the photoresist to be spun and to form thin layers over the wafer surface. Finally, the sensitizer, or inhibitor, controls the photochemical reaction in the .
Photoresists can be classified as positive or negative. In the positive photoresists, the photochemical reaction that occurs during exposure weakens the polymer, making it more soluble to the developer, so the positive pattern is achieved. The mask, therefore, contains an exact copy of the pattern, which is to remain on the wafer, as a stencil for subsequent processing. In the case of negative photoresists, exposure to light causes the polymerization of the photoresist, and therefore the negative resist remains on the surface of the substrate where it is exposed, and the developer solution removes only the unexposed areas. Therefore, masks used for negative photoresists contain the inverse or photographic “negative” of the pattern to be transferred. Both negative and positive photoresists have advantages and disadvantages. The advantages of negative photoresists are to silicon, lower cost, and a shorter processing time. The advantages of positive photoresists are better resolution and thermal stability. Fig. 11.2 shows the result of the exposure of both photoresists. As can be seen, the same mask produces complementary structures.
Figure 11.2. Negative and positive photoresists are exposed throughout a mask. The result of both cases is complementary.
To improve the adhesion of the photoresist, a prior preparation of the substrate is usually carried out. A promoter, which is a material that gives rise to of the photoresist to the substrate, is applied. Some promoters are deposited by spinning, whereas others are deposited by introducing the substrate to a desiccator with the promoter, so that it can be adhered by evaporation.
